The present invention relates to a titration device and method and, more particularly to one using electrolytic titrant generation.
Determining the total acidity or alkalinity of a sample is now a routine part of characterizing a sample in many industrial laboratories. A simple way to pursue this measurement uses a volumetric titration apparatus including a volumetric burette from which the appropriate titrant, usually acid or base titrant, is added to a stirred sample while monitoring sample characteristics such as pH or conductivity. From the characteristic plot of detector response versus volume of the titrant and the associated endpoint, the equivalents of acid or base in a given sample is determined. The field of titration is over 100 years old, and there is significant literature covering various aspects of titration. There are also several books in basic analytical chemistry that cover the basic aspects of titration theory and the determination of the end point (For example Chapter 3 entitled Gravimetric and Volumetric Analysis, Analytical Chemistry HandBook, by John A. Dean, McGraw Hill Inc, 1995).
Modern titrators use an automated means of dispensing the liquid titrant, for example, by using a motorized syringe. The dispensing of the titrant is done in a constant mode with each increment having a constant volume or in a dynamic mode where the titrant is added in large aliquots and near the end point the addition frequency is reduced to get an accurate determination of the end point. The dynamic mode expedites the analysis and reduces the analysis time. Despite the advances, modern titrators still use liquid titrant reagents that need to be frequently prepared and replenished to ensure no degradation or build up of contaminations. For example titration with a base titrant results in a carbonate error with an indicator with a basic transition point (phenolphthalein). For basic titrants proper precautions during preparation and storage needs to be taken.
Coulometry is an established methodology in the field of titration. Typically, in volumetric titration the volume of liquid titrant required to neutralize the acid or base in a sample is measured. In a coulometric titration the titrant is generated electrochemically and the quantity of electric charge is measured. Coulometry titration could be classified as primary and secondary titration. In the primary titration methods the titrant is directly derived from the electrode. Examples of this type of electrodes are silver metal, mercury, or mercury amalgam or electrodes coated with silver-silver halide and they generate the ions required for the titration for example silver metal anode forming silver ions that can be used to titrate chloride. Secondary titration methods are much more popular and use an intermediate ion generated from a precursor that is added to the supporting electrolyte. The intermediate ions must be generated with 100% current efficiency and must react rapidly and stoichiometrically with the substance being determined. For example during the coulometric titration of Fe (II) to Fe (III), the method will not be 100% current efficient unless excess Ce (III) is added as the precursor to the supporting electrolyte sulfuric acid.
The current in a coulometric titration is usually maintained constant and by monitoring the time in seconds, the number of coulombs required to titrate a species and hence the number of equivalents is easily derived. Detection of the end point of the titration occurs via conventional means such as color change from addition of an indicator. Other means such as using an amperometric, pH or conductivity detection is also routinely used.
A review of literature indicates there are several patents and publications relating to various aspects of titration. U.S. Pat. No. 2,744,061 discloses a titration apparatus in which the reagent is prepared in a separate cell from the titration cell and the reagent is then introduced into the titration vessel. Such a scheme produces acid and base from electrolysis of 1 M sodium sulfate solution and one or the other stream could be introduced for titration. The method suffers from a) sample dilution errors b) need to control the flow of the sodium sulfate feed c) addition of salt solution to the sample since not all of the electrolyte was used for the formation of acid or base and this would limit the detection to detectors that are sensitive to the presence of salt. For example conductivity detectors cannot be used in the above scheme because the conductivity of the 1 M salt solution will overwhelm the detector response making it difficult to detect small changes in conductivity from the presence of small amounts of acid or base.
Another version of a coulometric titration apparatus using the silver ions generated from a silver anode is discussed in U.S. Pat. No. 3,032,493. Yet another version of a titration device is discussed in U.S. Pat. No. 3,308,041. This is a continuous titration device for process applications. The titrant is generated coulometically and then mixed with the liquid sample to form reactant product which is directed to the sensing means for detection. Also some feedback mechanism between the sensing and the titrant is discussed that allows control of the generated titrant.
U.S. Pat. No. 3,856,633 discloses a concentration measurement technique that uses coulometrically generated silver ions from a silver anode and a platinum cathode in the presence of sample cyanide ions. Silver ions react with the cyanide ions lowering the free silver ion availability which is then monitored across a silver sulfide membrane by a potential measurement. The free silver ions unreacted with the sample are on one side of the membrane, and a standard with a known quantity of silver ions is on the other side of the membrane. From the developed potential a detection scheme was available for the sample ions. In another embodiment the silver sulfide membrane was placed between the silver anode and the platinum cathode to protect the silver anode from exposure to reactive species such as nitrate in the sample.
U.S. Pat. No. 4,007,105 discloses a titration apparatus for coulometric titration of chloride in blood samples that uses a silver anode and a platinum cathode in an acid electrolyte medium. The silver ions generated in proportional to an applied current reacts with the chloride in the sample forming silver chloride and the titration is monitored using a pair of amperometric electrodes. Also disclosed is the arrangement of the electrode cells.
In many of the above approaches since the oxidation or reduction of the analyte ions occur at the generation electrodes, as the solution is depleted of analyte an effect called concentration polarization occurs. The potential must therefore increase to maintain the constant current operation. This higher potential in turn results in lower current efficiency due to undesired side reactions on the electrode surface.
While conventional coulometric titration require all of the analyte to be reacted, a version of this approach called “flash titration” where only a portion of the sample stream in the vicinity of the electrode is titrated significantly reduced the overall titration time. Since the methodology is a diffusion regulated process, the method is sensitive to varying diffusion rates of various sample acids and bases, temperature and sample viscosity. A calibration step called the matrix adjustment factor is needed to compensate for the above effects.